Protection of power distribution systems involves detecting, locating and initiating the removal of a fault from the power system. Identifying the location of faults is an important process. Protective relays are extensively used for major protective functions. Protection systems and circuit breakers are installed at strategic locations along the feeder for the purpose of detecting faults that cause excess current to flow and to automatically disconnect them from the source. Manual operations are usually required to isolate the faulty section and this can take several hours during which time many customers are without electricity.
Time grading techniques are often used to minimize the number of customers disconnected when isolating a fault. Time graded protective systems have protective devices in successive zones that are arranged to operate in times which are graded through the sequence of equipment so that upon the occurrence of a fault only those relevant to the faulty zone complete the tripping function. A disadvantage of time grading schemes is that they are slow to identify faulted zones and due to the time separation required and it is assumption that there is a single source feeding the system. This method is not suitable when dealing with multiple variable sources like wind and solar generation and will require continual adaptive setting changes in real time.
A problem of slow isolation and restoration of distribution power system faults leads to lengthy unwanted power outages. It is today still typical for these actions to be performed by linemen driving in trucks from switch to switch. Power outages for the most part leads directly to loss in revenues to Utilities, fines imposed by regulators on utilities and loss in production on the part of electrical consumers like factories that are dependent on a constant supply of electrical power.
The problem to create a smart distribution grid has been hampered by the use of old communications systems, protocols and communication infrastructures. The problem that we have today in distribution feeders is that the systems do not have the capability to inelegantly isolate faulted sections and restore these feeders after a fault has been cleared from the system. The procedure of isolation and restoration have for a large extent been a slow manual process either carried out through manual switching in the field by line man or utility technicians/electricians or switching remotely by an operator through a Distribution control center connected to very simple RTU of small PLC type of field device. Several attempts have been made to create centralized systems that would gather data for the field devices and then process this information centrally in logic equations to perform so called “distribution feeder automation”. These systems are all notoriously slow to gather the data from the field and present in to the Control Center. It could take many minutes before a fault could be isolated through repeated switching on to a fault using an elimination process to isolate the fault.
Current Systems act extremely slowly and my take minutes to react to changing system conditions. This is undesirable and cause unwanted power outages while faults are being isolated or restored. Manual switching at the switch or remote switching from a SCADA control system is commonly used. Non-communicating systems called loop automation systems are used with reclosers. These systems act on presence of voltage measured at the switching point and local logic. Current smart systems use specially adapted communication protocols like DNP3 to communicate between devices to achieve restricted logic. These systems are vendor specific and are not open for adaptation or modification by the user as his needs might change. These systems are also restricted to a small number of switching devices. The system also relies on a master device to perform desired logic.
A number of systems exist dealing with fault detection of circuits in power distribution networks such as those described in U.S. Pat. Nos. 6,603,649; 6,687,573; 6,697,240, 7,636,616; 7,773,360; 2008/0024142; and 2009/0290275, all of which are incorporated herein by reference.
There is a need in the art for a scheme that can clearly detect a faulted zone in a shorter time and with less impact on the connected power grid and that is also immune to the effect of the introduction of distributed generation in distribution feeder networks. There is also a need in the art for a system that will not require a master or token type of arrangement.
The present invention addresses those needs.